HOW IT WORKS

HOW IT WORKS; A Printer That Paints With Bursts of Laser Light

By MATT LAKE

Published: February 28, 2002

LIKE the personal computers they are tethered to, laser printers first appeared in the 1970's and took off over the next two decades. More than 60 million are now installed in the United States, making laser models the most popular printing technology in the workplace.

But although laser printers are tied to the rise of personal computing, they arose from research conducted before World War II. In 1938, a patent lawyer, Chester Carlson, discovered a dry printing process that he called electrophotography, which used light-sensitive drums and a sooty powder instead of ink.

Mr. Carlson spent almost a decade trying to interest major corporations in his invention before the small Haloid Company finally picked it up. Haloid renamed the process xerography (from Greek for dry writing) and eventually renamed itself after the technology, becoming Haloid Xerox and later, the Xerox Corporation.

Over the years, the basic concept for what is now largely known as photocopying has remained the same. An image of the page to be printed is formed by altering the electrostatic charges on the light-sensitive drum; then electrically charged toner is picked up by the drum and transferred to the paper, where it is fused by heat. A new uniform charge is then applied to the drum and the cycle is repeated.

What has changed is how the image gets on the drum. A traditional photocopying machine uses light that reflects off the original document placed on the glass. In a laser printer, the ''original'' is simply a data file. A computer chip processes the data, and a laser light source paints it on the drum.

The technologies used in modern laser printing were developed during the 1970's by Xerox and a rival, Canon. The companies took different approaches to the technology to match their different approaches to business. Xerox focused on large corporations that bought products and service contracts. Canon specialized in equipment that its customers could maintain themselves, so it developed a replaceable cartridge that contained the toner, and the printing drum. It then developed its own printers and licensed the technology to other companies.

One of the licensees, Hewlett-Packard, began using Canon print engines in 1984 in its LaserJet line, which went on to capture the largest share of the laser printer market.

The most complicated element in laser printing is the image processing: translating computer documents containing words, graphics and images into precisely placed lines, curves and lettering on the page. With most printers, this takes place in a chip inside the printer called the raster image processor, and the printing jobs are lined up in RAM chips or on hard disks.

But some printers cut corners (and costs) by using the computer's chips and operating system to translate the pages for printing instead of using expensive processors and memory inside the printer to do so. Such models, which are called G.D.I. printers after the Windows Graphics Device Interface or QuickDraw printers after Apple's equivalent, are typically inexpensive because they do not need to include the processors. But they tend to occupy more of the computer's time.

Laser printers have two big rivals: inkjet and L.E.D., or light-emitting diode, printers. L.E.D.'s are cousins to the laser printer: they, too, print from a light-sensitive drum, but use a row of diodes instead of a single laser, which makes them cheaper to manufacture. Early L.E.D. models tended to shift out of alignment, which made some buyers wary. Laser printers can realign themselves more easily.

Inkjet printers dominate the home computing market. They are cheaper and often print text almost as well as laser printers.

But laser printers still have the upper hand for anyone who prints black-and-white documents in large volume. They print faster, the printouts will not smear when wet, and the pages cost less to print than those from inkjet printers. Modern inkjets typically use more than 5 cents' worth of black ink per page, and sometimes as much as 11 cents. With the latest laser printers, the cost per page is 2 1/2 to 4 cents.

Color remains the big stumbling block for laser printers. Color laser printers can cost 10 times the price of color inkjets and often lack the speed advantage of black-and-white laser printers. That is because many color laser printers use a single laser assembly and drum and send each page through four times, for each signature color (cyan, magenta, yellow and black). Newer models use four assemblies, making the machines faster but more costly.

But prices of color laser printers have dropped significantly. Minolta's Magicolor 2200 and Lexmark's Optra C710 line now cost less than $1,000, and several other companies make color laser printers priced at less than $2,000. Color laser printers can print a color page for 6 to 12 cents, while all-color inkjet pages can cost as much as 20 cents each.

Laser printers have also given something back to the technology that spawned them. Modern photocopiers no longer use Carlson's original technique of reflecting light onto the drum. Modern photocopiers create digital copies by scanning and digitizing the original document and storing it on RAM in the photocopier. Then they laser-print the digital images.

Drawing (Frank O'Connell/The New York Times) Chart: ''Inside an Office Workhorse'' Laser printing is a hybrid of ideas and technologies. At its heart is an electrostatic technique developed 60 years ago for photocopying, but grafted onto this is laser and computing technology developed in the 1970's. The result is crisp and dry printed pages that are literally hot off the presses. 1) SENDING THE PRINT JOB With a click on ''Print,'' a computer's operating system translates a document into a series of printing instructions. To perform the translation, the computer uses driver software that speaks the same page description language as the printer. 2) BUILDING THE PAGES In many printers, the print job goes to a processor, which determines where each dot must go on the page to create the printed document. The instructions are stored in the printer's random access memory. 3) DRAWING THE PAGES Using the processor's instructions, a laser flashes at precise intervals into a rapidly spinning many-sided mirror. This reflects tiny dots of light onto a drum covered with a light-sensitive material that carries an electrostatic charge. Where the light hits it, the charge is altered. In this way, the laser ''paints'' a mirror image of the document on the drum, line by line, as the drum revolves. 4) ADDING THE INK Toner, a fine sooty powder that also carries an electrostatic charge, is picked up by a cylinder from a reservoir. The toner is then transferred to the drum, but only on the charged areas. 5) TRANSFERRING ONTO THE PAGE As the printer draws paper through a system of rollers, a transfer charging roller adds a strong charge to the paper that draws the toner off the drum as the paper passes through. 6) ERASING THE DRUM As the drum rolls on, any residual toner is scraped off by a rubber blade. A primary charge roller applies a uniform negative charge to the drum, preparing it for the next part of the page to be drawn by the laser. 7) FIXING THE PAGE The printer fixes the powdery toner in place on the paper using a fuser roller that is heated to about 350 degrees Fahrenheit and a pressure roller. THE CARTRIDGE In many printers, most of the drums and rollers are part of a cartridge, which also contains the toner reservoir. Toner is a mixture of plastic particles and iron oxide particles (which can hold a charge). (Sources: Canon U.S.A.; Hewlett-Packard)